JNER JOURNAL OF NEUROENGINEERING AND REHABILITATION Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Open Access RESEARCH © 2010 Magalhães and Kohn; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and repro- duction in any medium, provided the original work is properly cited. Research Vibration-induced extra torque during electrically-evoked contractions of the human calf muscles Fernando H Magalhães* † and AndréFKohn † Abstract Background: High-frequency trains of electrical stimulation applied over the lower limb muscles can generate forces higher than would be expected from a peripheral mechanism (i.e. by direct activation of motor axons). This phenomenon is presumably originated within the central nervous system by synaptic input from Ia afferents to motoneurons and is consistent with the development of plateau potentials. The first objective of this work was to investigate if vibration (sinusoidal or random) applied to the Achilles tendon is also able to generate large magnitude extra torques in the triceps surae muscle group. The second objective was to verify if the extra torques that were found were accompanied by increases in motoneuron excitability. Methods: Subjects (n = 6) were seated on a chair and the right foot was strapped to a pedal attached to a torque meter. The isometric ankle torque was measured in response to different patterns of coupled electrical (20-Hz, rectangular 1-ms pulses) and mechanical stimuli (either 100-Hz sinusoid or gaussian white noise) applied to the triceps surae muscle group. In an additional investigation, M max and F-waves were elicited at different times before or after the vibratory stimulation. Results: The vibratory bursts could generate substantial self-sustained extra torques, either with or without the background 20-Hz electrical stimulation applied simultaneously with the vibration. The extra torque generation was accompanied by increased motoneuron excitability, since an increase in the peak-to-peak amplitude of soleus F waves was observed. The delivery of electrical stimulation following the vibration was essential to keep the maintained extra torques and increased F-waves. Conclusions: These results show that vibratory stimuli applied with a background electrical stimulation generate considerable force levels (up to about 50% MVC) due to the spinal recruitment of motoneurons. The association of vibration and electrical stimulation could be beneficial for many therapeutic interventions and vibration-based exercise programs. The command for the vibration-induced extra torques presumably activates spinal motoneurons following the size principle, which is a desirable feature for stimulation paradigms. Background Percutaneous electrical stimulation applied directly over the human muscle can elicit contractions by two distinct mechanisms [1,2]: peripheral and/or central. The more common is by the direct stimulation of the terminal branches of motor axons, considered to be of peripheral origin, and hence the generated torque has been called peripheral torque (PT). Alternatively, the stimulation may elicit action potentials in large sensory afferents (favored by the use of low-intensity, wide-pulse-width, high-fre- quency stimulation [1]) which can synaptically recruit α- motoneurons in the spinal cord. The generated torque has been sometimes called central torque, and has the important feature of being associated with motor unit recruitment in the natural order, starting with the fatigue-resistant units [2-4]. This has obvious beneficial implications for neuromuscular electrical stimulation * Correspondence: fhmagalhaes@leb.usp.br 1 Neuroscience Program and Biomedical Engineering Laboratory, Universidade de São Paulo, EPUSP, PTC, Avenida Professor Luciano Gualberto, Travessa 3, n.158, Butanta, São Paulo, SP, Brazil † Contributed equally Full list of author information is available at the end of the article Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 2 of 16 (NMES), functional electrical stimulation (FES) and other therapeutic interventions. The excitatory input to the motoneurons provided by the sensory volley can produce surprisingly large forces and an unexpected relation between stimulus frequency and evoked contractions [5,6]. For example, when brief periods of high frequency (e.g. 100 Hz) electrical stimulation were delivered on top of a longer train of stimuli kept at a lower frequency (e.g. 25 Hz), there was a large increment in force attributed to the central mechanism. When the stimulation returned to 25 Hz the force remained unexpectedly high [2,5,6]. That is, during a burst-like pattern that alternated periods of 25 and 100 Hz stimulation, more force was generated after the high-frequency burst than before it, despite the similar stimulus frequency and intensity [2,5,6]. In some cases, these sustained forces observed following the high- frequency-bursts could continue even after the end of the stimulation period (i.e. when any stimulus was already turned off) [5]. The "extra force" associated with the central torque, is not present when a nerve block is applied proximal to the stimulation site [5-7], but remains present both in com- plete spinal cord-injured [5,8] and healthy sleeping sub- jects [5], which confirms the involuntary and central origin of the phenomenon. This "extra", self-sustained contraction produced by the involuntary central mechanism, which will be named here "extra torque (ET)", is developed in addition to the torque due to motor axon stimulation [2,5,6,9], and can be quite large, up to 42% of the maximal voluntary con- traction (MVC) [6]. Such ET has been proposed to be due to an increase in firing rate and recruitment of new motoneurons through either the development of plateau potentials and/or post-tetanic potentiation (PTP) [5,6]. PTP would increase the release of neurotransmitter from the large sensory axons through high frequency stimula- tion, thus leading to the activation of higher threshold motoneurons [10]. The sensory volley could also activate motoneuron plateau potentials, trough the opening of voltage-gated L-type Ca ++ channels (for example), thus generating persistent inward currents (PICs) that would produce continuous depolarization (plateau potential) [11-13] and consequently self-sustained motoreuron dis- charge that may be dissociated from the stimulus pulse [9] The contraction generated by electrically evoked affer- ent input to the spinal cord, which is responsible for trig- gering the ET through a central mechanism, resembles that generated during tonic vibration reflex (TVR), which develops when vibration is applied to a muscle or its ten- don. Both mechanisms are triggered by large-diameter afferents, may often outlast the stimulus, develop in a slow fashion and are involuntary but can be abolished by volition [6,14,15]. Furthermore, studies performed in ani- mal preparations have suggested that the activation of plateau potentials also plays a role in the generation of TVR [16]. However, more direct experimental evidence that the firing of human motor units is determined by intrinsic properties such as plateau potentials has been obtained only for a low level voluntary activation of a muscle [17- 19] The present work had as a goal to investigate if vibra- tion is also able to generate large magnitude self sustained ETs, markedly larger than the PT evoked by low-fre- quency electrical stimulation. More specifically, we aimed to investigate whether vibration may evoke self- sustained forces at levels comparable with those ETs pre- viously shown in response to high-frequency electrical stimulation [2,5,6]. In addition, we sought to investigate if the vibratory stimuli caused an increase in the motoneuron excitability, which could lead to ET from the innervated muscle. In this regard, the F wave is a late response that occurs in a muscle following stimulation of its motor nerve, evoked by antidromic reactivation ("backfiring") of a fraction of the motoneurons and is sensitive to changes in motoneu- ron excitability [20]. In contrast to the H-reflex, which is dependent on presynaptic inhibition and homosynaptic depression, the F response is not elicited by a Ia volley [21], and would therefore be a useful method for assess- ing the excitability of the motoneuron pool in this experi- ment. Although the use of F waves for assessing motoneuron excitability is controversial [21,22], F waves reflect motoneuron excitability in a general way [23]. Finally, it is important to emphasize that there are important differences between the effects of electrical and vibratory stimuli. An obvious difference is the lack of antidromic activation of motoneuron (and sensory) axons during vibration. This means that there is no collision (and annihilation) of reflexively generated action poten- tials and the antidromic action potentials. In addition, the temporal dispersion of Ia afferent volleys in the tibial nerve induced by Achilles tendon percussion is much greater than that of electrically induced volleys, which may lead to differences in central transmission [24]. Fur- thermore, group II, Ib and cutaneous afferent discharges induced by electrical stimulation of the tibial nerve are different from those induced by Achilles tendon percus- sion [25,26]. Hence vibration's ability to evoke extra torques similar to those obtained in response to wide pulse width, high frequency electrical stimulation cannot be easily predicted. Methods Assessing ET Generation Six male subjects (30 ± 5.3 (SD) age, ranging from 26 to 37 years) volunteered to participate in this study. The Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 3 of 16 experiments had approval by the local ethics committee and were conducted in accordance with the Declaration of Helsinki. Each subject signed an informed consent document. Subjects were seated on a customized chair designed for measuring ankle torque during isolated isometric plantarflexion contraction. The hip, knee and ankle of the right leg were maintained at 90° with an adjustable metal bar placed over the anterior distal femur, superior to the patella and fixed to the chair, avoiding any movement of the thigh. The right foot (all subjects were right-footed) was tightly fixed to a rigid metal pedal so that its axis of rotation was aligned with the medial malleolus. A strain gauge force transducer (Transtec N320, Brazil) was attached to the pedal for isometric torque measurements. At the beginning of the session, each subject's maximal voluntary force during plantarflexion was determined. Subjects were asked to perform three MVCs of the tri- ceps surae (TS), with 2 min rest between each trial. The maximum force value achieved across the three trials was taken as the MVC force value. All measurements in this paper are expressed as a percentage of the MVC (and hence we use the terms torque and force interchange- ably). Flexible silicon stimulating electrodes (10 cm long × 5 cm wide) were fixed over the subjects' right calf muscle. The proximal electrode was positioned midway across the two portions of the gastrocnemius muscles, ~10-15 cm distal to the popliteal fossa. The distal electrode was placed over the soleus, just below the inferior margin of the two heads of the gastrocnemius muscle. A DIA- PULSI 990 stimulator (Quark, Brazil) was driven by a computer that controlled the delivery of rectangular pulses of 1-ms duration. A single burst consisting of 5 pulses at 100 Hz was used in order to set the stimulus intensity, progressively adjusting the current until the peak ankle torque produced by such stimuli reached ~5% of the subject's MVC value [5]. It has been previously demonstrated that such intensity is optimal for generat- ing marked ETs in the TS muscle group in response to burst patterns alternating higher and lower frequencies of electrical stimulation (e.g. 20-100-20 Hz) [2,6]. The Achilles tendon of the right leg was stimulated mechanically by means of a LW-126-13 vibration system (Labworks, USA), consisting of a power amplifier and a shaker (cylindrical body, with diameter 10.5 cm and length 13.5 cm). The shaker was fixed to the bottom structure of the chair, so that the tip of the shaker (round- shaped plastic tip, 1 cm diameter) was pressed against the Achilles tendon in order to keep a steady pressure and a fixed position on the tendon. A LabView system (National Instruments, USA) was utilized to generate either 100-Hz sine waves or gaussian white noise signals with 2-s dura- tion, which were delivered to the input of the shaker's power amplifier in order to obtain the desired mechanical stimulation. An ADXL78 accelerometer (Analog Devices, USA) was attached to the movable part of the shaker in order to monitor the parameters of the mechanical stim- uli. Eight 2-s-bursts of 100-Hz electrical stimulation sepa- rated by 2 s of 20-Hz stimulation (starting with a 2-s and ending with a 3-s period of 20-Hz stimulation) were ini- tially applied. Such a pattern (named here stimulation pattern 1), is similar to that successfully utilized by previ- ous studies [2,5-7] in order to observe ETs generated by high frequency bursts of electrical stimulation. It is also being included here in order to assure inter-studies repeatability as well to compare, in the same sample of subjects, ETs triggered by electrical stimulation with those triggered by vibration. Additionally, two different patterns of coupled electrical (20 Hz, rectangular 1-ms pulses) and mechanical (either 100-Hz sinusoidal or white gaussian noise pattern) stimulations were utilized, and will be named in the text as stimulation patterns 2 and 3, respectively: 35 s of 20 Hz electrical stimulation together with 8 intermittent bursts of mechanical stimuli of 2 s duration, starting at 2 s and finishing 3 s before the end of the electrical stimuli (stimulation pattern 2); and 35 s of alternated 2 s of electrical and 2 s of mechanical stimuli, resulting in 8 bursts of mechanical vibration (stimulation pattern 3). Thus, 3 different stimulation pat- terns were utilized, and will be referred in the text as pat- terns 1 to 3 (see figure 1, figure 2 and figure 3 for examples). In addition, for control purpose, each subject completed two 35 s trials of 20-Hz electrical stimulation. In a few subjects, three 2-s bursts of 100-Hz sinusoidal vibration were alternated with 2-s 20 Hz electrical stimu- lation trains, starting with 2-s and ending with a long train (23 s) of 20 Hz electrical stimuli (see figure 4). Such paradigm was used to evaluate the time decay of the evoked ETs during the last 23 seconds of 20 Hz electrical stimulation alone, as well as to compare its responses with those evoked by TVRs generated by three 2 s of 100 Hz sinusoidal vibration bursts applied without electrical stimuli (see figure 4). These paradigms will be named "additional investigations" in the results section. When the paradigm involved only vibratory stimula- tion, the EMG signals from the soleus muscle in response to vibration were acquired simultaneously with the sig- nals from the force transducer and the accelerometer. The EMG signals were amplified and filtered (10 Hz to 1 kHz) by a MEB 4200 system (Nihon-Kohden, Japan). Round-shaped surface electrodes (0.8 cm diameter, prox- imal-distal orientation, with an inter-electrode distance of 2 cm) were positioned over the soleus muscle, the most proximal contact being 4 cm beneath the inferior margin of the two heads of the gastrocnemius muscle. A ground electrode was placed over the tibia. Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 4 of 16 The peak-to-peak acceleration of the 100 Hz sinusoidal vibration used in this study was 200.g in the average (200 times the acceleration of gravity). This corresponded to a RMS value around 70.g and a peak-to-peak displacement of the tip of the shaker around 5 mm. The RMS value of the Gaussian white noise vibration was around 27.g (see inset of figure 2 for a visualization of the white noise amplitude distribution and spectrum). The subjects were asked to relax completely, not mak- ing any voluntary effort during the stimulation trials. Each subject completed 8 trials of each stimulation para- digm described above with an inter-trial interval of ~90 s. A program written in the Workbench environment (DataWave Technologies, USA) was used to deliver trig- ger pulses in order to synchronize the occurrence of each 2 s of mechanical (sinusoidal or noise) bursts and the start of the torque, EMG and accelerometer data acquisi- tion (sampled at 5 KHz). The same program controlled the pulses delivered by the electrical stimulator. The evoked forces generated by the stimulation pat- terns utilized here initially showed a peripheral compo- nent, presumably originated from the direct stimulation of motor axons in response to the 20-Hz electrical stimu- lation. Subsequently, a central component was observed, reflexively evoked from either high frequency electrical stimulation [2,6] or vibration bursts. Finally, the so called ET emerged, defined as the additional torque developed over the PT value, triggered by the central mechanism, thus observed after the end of a high-frequency electrical stimulation or vibratory burst. The outcome variables of interest in this particular study were the PT and the ET. To quantify them, we adapted a method proposed by Dean and colleagues [2]. PT was defined as the torque level produced during the first 2 s of the 20-Hz-stimula- tion initially applied (before the delivery of any 100-Hz electrical stimulation or vibration bursts), and ET was quantified as the additional torque measured during the following periods of 2 s with no stimuli besides the basal 20 Hz electrical stimulation. To quantify the torque pro- duced during a given time period, the average torque was calculated during the most stable 0.5-s interval contained in that period (i.e. with the smallest coefficient of varia- tion). Figure 1 Peripheral and extra torques generated by stimulation pattern 1. A) Schematic representation of stimulation pattern 1 showing the time course of alternating 2-s of 20-Hz and 100-Hz bursts of electrical stimulation. B) Average plantarflexion torque as a function of time (n = 8, thick line) with SD shown in light shade. Bars (thin line) represent the values of peripheral torque (PT) and extra torques (ETs, means ± SDs). Note that the ET values are the increments with respect to the PT value. The eight extra torque values generated by the series of 100-Hz bursts are labeled ET1 ET8. Data are from a representative subject. D) Average extra torques (± SEMs) representing group data (n = 48). Asterisks indicate extra torque values sig- nificantly different from zero (p < 0.05). Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 5 of 16 Assessing Motoneuron Excitability The experiments were performed on three healthy men (30 ± 4.7 (SD) age), with informed consent and the approval of the local ethics committee. These subjects had previously participated in the experiments for assess- ing ET generation and each had exhibited significant ETs during all the stimulation patterns utilized (see Results). Additionally, these subjects had also shown increased ETs when additional vibratory bursts were delivered (see Results, figure 1, figure 2, figure 3 and figure 4). All pro- Figure 2 Peripheral and extra torques generated by stimulation pattern 2. At the top, the first two graphs show the amplitude histogram and the absolute value of the FFT of the Gaussian white noise acceleration signal and the third graph shows the absolute value of the FFT of the sinusoidal acceleration signal measured at the tip of the shaker. A) Schematic representation of stimulation pattern 2, showing the time course of 8 intermittent bursts of vibratory stimuli of 2 s duration (rectangular boxes) together with a constant background 20 Hz electrical stimulation. B) Average plantar- flexion torque as a function of time (n = 8, thick line) with SD shown in light shade. Bars (thin line) represent the values of peripheral torque (PT) and extra torques (ETs, means ± SDs). The eight extra torque values generated by the series of 100-Hz bursts are labeled ET1 ET8. Data are from a repre- sentative subject. C) The same as in B but for the white noise vibratory bursts instead of the 100-Hz sine wave bursts (both B and C are data from the same representative subject). D and E) Average extra torques (± SEMs) representing group data (n = 48) for the stimuli utilizing 100 Hz sine waves (D) and white noise (E). Asterisks indicate extra torque values significantly different from zero (p < 0,05). Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 6 of 16 cedures and apparatus were identical to those previously described here, except for the stimulation techniques to evoke F waves and the stimulation paradigms employed (i.e. stimulation patterns). In order to record the M and F waves evoked in response to supramaximal tibial nerve stimulation, the EMG signals from the right soleus muscle were acquired. Round-shaped surface electrodes (0.8 cm diameter, prox- imal-distal orientation, with an inter-electrode distance of 2 cm) were positioned over the soleus muscle, the most proximal contact being 5 cm below the inferior margin of the two heads of the gastrocnemius muscle (just below the distal silicon stimulating electrode). A ground elec- trode was placed over the tibia. The EMG signals were fil- tered from 100 Hz to 1 kHz, the highpass cutoff being chosen higher than usual to attenuate the stimulus arti- facts from the 20-Hz percutaneous electrical stimulation. F waves were evoked by supramaximal electrical stimu- lation of the posterior tibial nerve (duration, 1 ms) by means of surface electrodes with the cathode (2 cm 2 ) in the popliteal fossa and the anode (8 cm 2 ) against the patella. At the beginning of each session, the maximal peak-to peak amplitude of the soleus compound muscle action potential (maximal M wave, M max ) was obtained. The stimulus intensity used to elicit F-waves was 180% of that required to elicit the M max . A sample of 10 responses were obtained at different times during the stimulation paradigm, both during the initial 2 s of 20-Hz electrical stimulation alone and during the 2 s of 20-Hz electrical Figure 3 Peripheral and extra torques generated by stimulation pattern 3. A) Schematic representation of stimulation pattern 3 showing the time course of alternated 2 s of electrical and 2 s of mechanical stimuli (rectangular boxes), resulting in 8 bursts of mechanical vibration. B-E) The same as in Figure 2, but with data regarding stimulation pattern 3 instead of 2. Data are from the same representative subject from figure 2. Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 7 of 16 stimulation after the delivery of 100-Hz vibratory sine waves stimulation (see figure 5). One supramaximal stimulus was delivered to the tibial nerve 50 ms after either one of the following pulses of a given burst of 20-Hz percutaneous electrical stimulation applied over the TS: 3 rd , 10 th , 20 th , 30 th and 40 th . This means that a supramaximal pulse was delivered at one of 5 possible latencies, one chosen at a time, being named here Time1 to Time 5, respectively (see, e.g., figure 5). In all the cases, stimuli used to evoke the F waves (test stimuli) terminated the stimulation session. That is, no further stimulation occurred after the delivery of a test stimulus. This avoided artifacts from the 20-Hz electrical stimulation to contaminate the signal. Therefore, an inde- pendent stimulation trial was performed for each F wave obtained. This ranged from a 200-ms stimulation (test stimulus delivered 50 ms after 3 pulses of percutaneous electrical stimulation at 20 Hz) to a 6.05 s stimulation (test stimulus delivered 50 ms after 2 s of percutaneous electrical stimulation at 20 Hz (40 pulses), preceded by 2 s of percutaneous electrical stimulation followed by 2 s of vibratory bursts). For control purposes, a sample of 10 responses at rest was also obtained. In addition, F waves were also obtained in response to a 2-s vibratory burst applied to the Achilles tendon alone (i.e. with no concomitant per- cutaneous electrical stimulation). For this, test stimuli (n = 10) were delivered to the tibial nerve 200, 550, and 1050 ms after the vibration (analogous to Time1 to Time 3). Statistical Analysis An Analysis of Variance (ANOVA) with repeated mea- sures and Bonferroni's post hoc tests (the latter per- formed where any significant main effects was pointed out by the preceding ANOVA test) were used to test whether each stimulation paradigm produced significant ETs and whether ETs differed from each other, both within single subjects and group data. Contrasts were performed at a 0.05 level of significance and ET was con- sidered to be significant when it was significantly greater than zero [2] (i.e., when the total torque value taken after each burst of high-frequency electrical or vibratory stim- ulation was significantly greater than that generated by the peripheral mechanism). All the analyses were per- formed using the statistical package SPSS 15.0 for Win- dows (SPSS, Inc., Chicago, Illinois). A descriptive analysis was used for the data regarding the F wave experiments. This was so because a sample of 3 subjects is not large enough for quantitative statistical tests. Figure 4 Responses to three vibratory bursts either alone or alternated with trains of electrical stimulation. A) Plantarflexion torque (seven superposed recordings) and EMG from the soleus muscle (typical recording) in response to three 2-s vibratory bursts (100 Hz sinusoidal waves) sep- arated by 2 s resting periods (no stimulation). The inset of the figure highlights the soleus EMG (black line) and the evoked plantarflexion force (gray line) on an expanded time scale (the two arrows indicate, respectively, the initiation of vibration and the monosynaptic response triggered by the first cycle of the vibratory stimulus). B) Plantarflexion force (seven superposed recordings) in response to three 2-s vibratory bursts (100 Hz sinusoidal waves) alternately applied with 20-Hz electrical stimulation (starting with 2s and terminating with 23 s of electrical stimulation). The two approximately constant responses (control values of force) correspond to the plantarflexion force evoked by 37 s of 20 Hz electrical stimulation alone (control stim- ulation). Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 8 of 16 Results Stimulation Pattern 1 Stimulation pattern 1, which alternated between 2-s- bursts of low frequency (20 Hz) and high frequency (100 Hz) percutaneous electrical stimulation (see above), gen- erated significant ETs (figure 1) in all the six subjects examined. The first high frequency burst was sufficient to evoke a significant ET. However, when additional bursts were delivered, two distinct responses could be observed: (1) in half of the subjects, a further increase in ET could be achieved by the subsequent 100 Hz bursts, until a pla- teau was reached by the third or fourth bursts (see figure 1B for example); the group data (6 subjects, 48 trials) showed the same behaviour (figure 1C); and (2) in the remaining three subjects, a significant decrease in torque was observed after the second or third bursts, i.e., the last five or six high frequency stimulation bursts were not able to generate significant ETs (i.e., not significantly dif- ferent from zero). This adds further information to previ- ous studies [2,9] that reported, in healthy populations, that some subjects do not generate any ET in response to wide-pulse electrical stimulation. Here, although all sub- jects were able to generate significant ET at the beginning of the stimulation, some of them could not maintain the extra force after the delivery of each high-frequency burst. Stimulation Pattern 2 In all subjects, a significant ET could be observed after the first 100-Hz burst of the vibratory pattern was applied Figure 5 Output plantarflexion force, M max and F-waves generated at rest and during periods of 20 Hz electrical stimulation before and af- ter the delivery of a vibratory burst. A) Schematic representation of a stimulation pattern showing the time course of two trains of 2 s of 20 Hz electrical stimulation separated by a single 2 s burst of vibration (100 Hz sinusoidal waves). B) Average torque as a function of time (n = 8, thick line) with SD shown in light shade. The arrows indicate the times (rest, Time 1, Time 3 and Time 5) when the M max and F-waves responses shown in (C) were obtained. C) M max -waves and F - waves recorded from the soleus muscle (10 superimposed repetitions are shown) at the times indicated by the ar- rows in B. Calibration bars for the M max are expressed in mV, while calibration bars for the F-waves are adjusted as a fraction of the corresponding M max (i.e. F-waves are normalized to the % of M max ). Data are from one representative subject. Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 9 of 16 to the Achilles tendon (during stimulation pattern 2) (see figure 2). Additional sinusoidal vibration bursts further increased ET values in four of the six subjects, achieving a steady value by the fourth or fifth bursts (figure 2B, for example). Again, this finding occurred also for group data (figure 2D, 6 subjects, 48 trials). In the other two subjects, the ET evoked by the first vibration burst either remained unchanged along the next 8 bursts or dropped to values not significantly different from zero after the fourth burst. Similarly, the first burst of the mechanical noise pattern applied to the Achilles tendon was sufficient to evoke sig- nificant ET in all subjects during stimulation pattern 2 (see figure 2) and the subsequent mechanical noise bursts increased ET further, until it reached a steady value by the fourth or fifth bursts. The group data followed this same behaviour (figure 2E). In two of the subjects (the same as before), a slight decrease in torque could be observed starting at the fifth or sixth bursts, but such a decrease was not significant. Stimulation Pattern 3 When the electrical stimulation was turned off during the application of the vibratory bursts (stimulation pattern 3), significant ETs could be observed in four of the six sub- jects examined, for both sinusoidal and white noise pat- terns, reaching a steady value around the fifth burst (figure 3B and 3C). This was similarly found for the group data, ETs achieving significance starting at the second vibratory burst (figure 3D and 3E). For the remaining two subjects, such stimulation did not produce significant ETs. Additional Investigations An example of three TVRs generated in response to three 2-s vibratory bursts (composed of sinusoidal waves) sepa- rated by 2-s resting periods (no stimulation) is illustrated in figure 4A. The upper signals (7 trials, 1 subject) show the evoked plantarflexion force waveforms and the lower signal shows the soleus EMG activity corresponding to one of the trials. The inclined arrow in the inset shows a single large EMG response at ~45 ms after the onset of the vibration, probably corresponding to the monosynap- tic reflex triggered by the first cycle of the vibratory stim- ulus. After a silent period of ~100 ms, the EMG activity began to gradually build up simultaneously to an increase in plantarflexion torque (gray curve), characterizing the slow development of the TVR. After the stimulation pat- tern ended, torque and EMG promptly returned to pre- stimulus levels, as they also did between the vibration bursts. When three bursts of 100-Hz sinusoidal vibration were alternately applied with 20-Hz electrical stimulation (figure 4B), the force exerted by the TS increased during the vibratory stimuli to levels comparable to those achieved by vibration alone. However, after the end of each vibratory burst, the plantarflexion force did not fall promptly to the control level (nearly constant responses in figure 4B). The force signal continued at high levels long after the vibratory bursts were turned off, gradually decreasing to the control values associated with the 20- Hz electrical stimulation. Motoneuron Excitability (M max and F waves data) At different times (Time 1 to Time 5) during the 20 Hz electrical stimulation, the F waves and M max evoked after the delivery of the vibratory bursts showed peak-to-peak amplitudes larger than those obtained before vibration (figure 5 and figure 6). After the delivery of a 2s vibratory burst alone (i.e, without the 20 Hz electrical stimulation), torque and EMG promptly returned to pre-stimulus levels (figure 7), similar to the responses observed in figure 4. Soon after the end of the vibration (i.e., at Time 1, 200 ms after vibration ended), clear increases in the peak-to-peak amplitudes of F waves and M max were observed (figure 7B and 7C). However, such increases did not persist (as they did when alternated with the 20 Hz electrical stimulation, figures 5 and figure 6), but returned to the control levels already at Time 2 or Time 3 (figure 7B and 7C). Discussion The results showed that vibration bursts (either high fre- quency sinusoids or white noise) delivered to the Achilles tendon can consistently increase the force generated by the TS muscle group while a basal train of 20-Hz electri- cal stimuli is applied to the TS. In most of the subjects, the vibratory bursts were able to keep the increased force even when the electrical stimulation was turned off dur- ing the vibration (alternating vibration with electrical stimulation). An additional investigation showed that the ET generation was accompanied by an increase in the amplitude of the F waves evoked in response to supra- maximal tibial nerve stimulation. The paradigm employed here involved no basal voluntary contraction and the ETs triggered by the central mechanism were of substantial amplitude. To our knowledge, this study pres- ents the first direct demonstration that markedly increased ETs, reaching values up to 50% MVC in differ- ent subjects, can be triggered reflexively by vibratory stimuli. In average, such increments were 180% of the PT value, ranging from no increment up to a nine-fold increase in torque over the PT value, in different subjects. Both presynaptic (PTP) and postsynaptic (PICs) mecha- nisms may contribute to these findings, due to the high frequency activation of large sensory afferents from the muscle spindles [27]. The experiments showed that vibratory bursts can gen- erate ETs at levels comparable with those additional Magalhães and Kohn Journal of NeuroEngineering and Rehabilitation 2010, 7:26 http://www.jneuroengrehab.com/content/7/1/26 Page 10 of 16 forces triggered in response to high-frequency electrical stimulation (see figure 1C, figure 2D, figure 2E, figure 3D and figure 3E). Extra torques could be generated either with or without a continuous background 20-Hz electri- cal stimulation applied simultaneously to the vibratory bursts (figure 2 and figure 3). When the electrical stimu- lation was turned off during vibration (in stimulation pat- tern 3, figure 3), the vibratory bursts caused a torque- interpolation by keeping on the mechanism for extra force generation. From an engineering point of view, the behaviour of the torque signals (compare figure 2 and fig- ure 4) show that the two inputs (an electrical stimulus train and the intermittent vibratory bursts) combine in a nonlinear way to generate the output torque as a function of time. The probable mechanisms are dealt with in the text ahead, but from an input-output point of view, the results indicate the importance of mixing the electrical stimulation (either basal or alternating) with the intermit- tent vibratory input to secure a change in the dynamics of the system and hence be able to obtain increased torque levels. The results of the current study are an extension of pre- vious reports [1,2,5,6,8,9] that suggested a central mecha- nism contributing to extra torque generation when surface NMES was applied to the subject's leg (with simi- larities to stimulation pattern 1 used in this study). In the new paradigms, the interpretations are perhaps simpler than in the NMES experiments of previous reports [1,2,5,6,8,9] because no antidromic activation of motoneuron axons occurs during the vibratory stimula- tion as may happen for electrical stimulation. In addition, the vibratory stimulation may induce motoneuron dis- Figure 6 M max and F-wave amplitudes measured at rest and during periods of 20 Hz electrical stimulation before and after the delivery of a vibratory burst. Peak-to-peak amplitude (n = 10, ± SEM) of the F-waves (black squares, expressed in the right axis as % of M max ) and the M max re- sponses (light gray circles, expressed in the left axis in mV) obtained at rest and at Time 1 to Time 5, both before and after the delivery of the 2 s vibra- tory burst (100 Hz sinusoidal waves). Note that during both the pre-vibration and the post-vibration phases the 20 Hz electrical stimulus train is being applied (see figure 5A). A, B and C are data taken from the three different subjects. [...]... induced by the vibratory bursts (e.g., up to 50% MVC) During the F wave study, a clear increase of the peakto-peak amplitudes of the Mmax was observed, both for the responses obtained during the first 2 s of 20 Hz electrical stimulation compared to rest and for the responses obtained during the 2 s of 20 Hz electrical stimulation after vibration compared to those obtained during the first 2 s of 20 Hz... response amplitudes to the corresponding Mmax obtained at the same joint angle and under the same experimental conditions Using such a normalization procedure, the present study showed a clear increase of the peak-to-peak amplitudes of the F waves for the responses obtained during the 2 s of 20 Hz electrical stimulation after vibration compared to those obtained during the first 2 s of 20 Hz electrical... [23] during a 20 Hz electrical stimulation applied after the delivery of brief vibratory bursts The increased excitability persisted during the whole time course of 20 Hz electrical stimulation (2 s) delivered after vibration The facilitation found at the level of the motoneuronal pool in the experiments with vibration occurred despite the possible development of presynaptic inhibition caused by the. .. by means of the F wave The concurrent low-frequency electrical stimulation was essential to make the extra torques induced by vibration observable When the same level of sinusoidal vibra- Page 13 of 16 tion stimulus was applied without the following 20-Hz electrical stimulation, the force promptly returned to the pre-stimulus level after vibration cessation On the other hand, self-sustained extra forces... differences in the size of the evoked afferent volley [10] Gorassini and colleagues [17] showed evidence of selfsustained firing in motoneurons of the intact human as vibration of the tibialis anterior muscle recruited an additional motor unit, beyond the one that was already firing due to the maintenance of a low level background volun- tary contraction (< 10% MVC) The recruitment of this second motor... provided by the sensory volley could lead to the development of plateau potentials in the motoneurons (a postsynaptic mechanism) A transient depolarization of sufficient amplitude and duration ("on" stimulus) can initiate a plateau potential [37], as it would be the case of TVRs evoked by the vibratory bursts in this investigation The substantial increment in the F wave amplitudes observed in the present... bursts (either sinusoids or white noise) delivered to the Achilles tendon could consistently increase the force generated by the triceps surae (TS) muscle group of able-bodied subjects while a basal train of electrical stimuli (20 Hz) was applied to the TS As the command for such extra force originated within the central nervous system, the resulting activation of spinal motoneurons would follow the size... besides other mechanoreceptors From a practical standpoint, this means that the vibratory bursts used to induce extra torque may be weaker than those required by the sinusoidal vibration used in this study (peak-to-peak displacement of the tip of the shaker around 5 mm, or, peak-to-peak acceleration of 200.g), and less specific than a 100 Hz stimulus This raises the possibility of activating such extra. .. in motor activity of the non-exercised contralateral limb [59] We also suggest that the underlying mechanisms of neuronal adaptations may be optimised by the use of stimulations techniques that favour the stimulation of sensory axons, leading to enhanced contractions mediated by a central mechanism, as obtained by the combination of vibratory and electrical stimulation Significant extra forces were... appeal, the real effectiveness of vibration and the physiological mechanisms involved in the adaptive responses to vibration exercise are still controversial [63] The present results suggest that vibration associated with electrical stimulation may provide an effective means of improving human muscle performance, since the electrical stimulation was shown here to be essential to "turn on" the vibration-induced . the top, the first two graphs show the amplitude histogram and the absolute value of the FFT of the Gaussian white noise acceleration signal and the third graph shows the absolute value of the. line) represent the values of peripheral torque (PT) and extra torques (ETs, means ± SDs). Note that the ET values are the increments with respect to the PT value. The eight extra torque values. cited. Research Vibration-induced extra torque during electrically-evoked contractions of the human calf muscles Fernando H Magalhães* † and AndréFKohn † Abstract Background: High-frequency trains of electrical